Practical Use of Thermal Capacitance

Last week, we talked about how researchers recently discovered pyramids long buried in the deserts of Egypt. Their “secret” technique is, in part, understanding the difference in thermal capacitance between the structures and the surrounding undisturbed soil. Just add heat from the sun and wait until it cools at night and then a difference in temperature becomes detectable.

Pour yourself a mug of coffee and ponder the topic of thermal capacitance! When you fill the mug, much of the heat from the beverage moves into the cold ceramic mug. This leaves the coffee cooler and the mug warmer. After you’ve finished the coffee, the mug stays warm for quite a while as it cools back to room temperature.

Similar techniques are being used to locate discontinuities in composite materials. Many aircraft are made of these lightweight structural materials composed of several layers bonded together. If the bonding fails, the structural integrity fails. By pulsing heat into the materials—typically using a powerful flash lamp—and observing the affect on surface temperature, these sub-surface anomalies can be located. The combination of “flash-active thermography” and sophisticated data processing routines, such as those developed by industry pioneers Thermal Wave Imaging makes it possible to test everything from the leading edge of the wing of the space shuttle to aircraft parts to blades in a power turbine.

These same types of techniques—heating or cooling materials and watching them as they go through the subsequent thermal transition—can work very well for less exotic applications. For example, a number of thermographers are using heat guns or lamps to look at marine hulls in order to locate damage from both moisture intrusion and impact. As with the aerospace composites, impact damage is often not visible to the eye but is disruptive enough to heat flow that it is obvious. Moisture trapped in composite assemblies is equally obvious in transition because it has a much greater thermal capacitance.

An area with a disbond shows up between structural members in a graphic-epoxy aerospace composite material. The flow of heat, pulsed into the material, is disrupted by the disbond leaving the surface above the defect warmer. (Courtesy Bob Ducar, Nordam)

One of the techniques I’ve discussed here before is the idea of “pulsing” heat (or cooled air) into a building to enhance thermal patterns. It is possible to walk into a building with no temperature difference from the inside to the outside and, by playing the “thermal capacitance game,” you will get decent results. The technique involves turning on the heat (or AC) for about twenty minutes and then waiting for a similar period of time. During this period of thermal transition, heat moves into (or out of) the sheetrock. Over both the framing and any voids, however, it moves differently compared to “normal” areas due to differences in capacitance. It is not necessary to heat the entire wall cross-section but only the surface. The resulting patterns emerge quite rapidly.

We actually have used similar “capacitance thinking” in a few instances where electrical systems could not safely be accessed while under load. In these cases the circuit was de-energized and the enclosure safely opened. Any thermal signatures associated with abnormal hotspots tend to stay warmer than normal points of connection, allowing for identification of the problems. Of course, the measured temperatures of the hotspots do not represent the maximum temperatures they might have been because, by the time we looked at them, they’d cooled significantly.

Even though these techniques based on thermal capacitance or “persistence” are fairly obvious to most of us, the fundamental concepts are not particularly intuitive. Go slowly or you’ll quickly find yourself in hot water or spewing hot air—puns not intended! While you may not be seeking buried pyramids, with care and good technique an understanding of thermal capacitance can help us see many other things that would otherwise be impossible or difficult to see.

I work in an electronic assembly center of printed circuit boards. Wondering if anyone has used the thermal capacitance theroy to help identify electrical test failures at this level. Please contact me if you have any information regarding my post.

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